Shellac finish are the easiest to remove followed by lacquered finishes. Never really hard when you can apply alcohol or a lacquer thinner to reactivate, dissolve or remove a finish. On a field grade gun sometimes you just need to remove the top portion of finish and leave what has filled all the pores in place. After all, many of these finishes are just a few layers deep with the top ones badly oxidized and holding a lot of dirt in them while the lower ones can be fairly decent.